Many types of machinery are subject to vibration, which may cause the loosening of threaded fasteners in the machinery. In many applications, such as turbine engines, a loosened fastener can cause catastrophic damage to the machinery. It is therefore necessary to lock the threaded fasteners in position so that they can not rotate in a direction which would tend to loosen and disengage them.
One locking system in the prior art utilizes two solid locking wires. Each of the fasteners to be locked has an aperture to receive a locking wire, and has one of the two wires passing through it aperture. The two wires are twisted together between fasteners and tensioned to prevent the fasteners from rotating in the loosening direction. The ends of the two wires are also twisted together and bent into a pigtail shape to prevent separation.
Instead of using two solid wires, a single solid wire may also be used to implement the twisted solid wire locking system. In that case the wire is first threaded through the aperture of the first fastener in the locking sequence. The wire is then doubled back around that fastener and the two halves of the wire are used in the same way as the two wires in the double wire system described above.
Another locking system in the prior art utilizes a single flexible multi-strand wire which is threaded through the aperture of each of the fasteners to be locked together. The wire is placed under tension and the tension is maintained by affixing a crimped-on body on each end of the wire to prevent the ends of the wires from going through the terminal fasteners. This single flexible wire system has many advantages over the twisted solid wire system, such as much easier installation and more uniform tension in the wire between fasteners.
Conventionally, after a locking system is put in place, a seal may be installed on the locking system. The seal is normally a malleable metal tag with an identification pattern impressed on it. The metal tag is normally installed on a section of the wire between two fasteners and crimped into position in such a way that the seal can not be removed without either destroying the seal or destroying the locking wire. In a solid wire locking system, a tag can also be crimped onto the twisted pigtail-shaped end of the locking wire to prevent separation of the ends.
The metal tag used as a seal can have a variety of configurations. For instance, a tag to be used on a multi-strand flexible locking wire may be a piece of short soft-metal tube which is to be slid on the wire during system installation between two fasteners. For a solid locking wire system the tag can be a small piece of sheet metal with two edges rolled up to form two parallel apertures. The tag is mounted between two threaded fasteners, with the solid locking wires threaded through the apertures and twisted before and after the tag.
In the prior art the identification mark on a metal tag used as a seal in a locking system is often created in the crimping process by using a crimp die which has an engraved pattern on the crimping surface. Crimping the tag with die leaves the pattern on the tag.
In summary, the seal in a locking system serves many purposes. One important purpose of using a seal is to provide a means of identification. For instance, the seal can be used for identifying the manufacturer, the individual assembler which has installed the locking system, or the time and location of the installation.
Another important purpose of using a seal is to allow the manufacturer to assure that a given device has not been tampered with. If someone has tampered with the device, they would have had to remove the fastener. In order to remove the fasteners they would have to remove the locking system which requires destroying either the locking wire or the seal. If the seal is configured in that it is not reusable, (i.e. the seal can not be removed from the locking wire intact and then used on another locking wire), the tampering cannot be disguised by installing another locking system on the device and attaching the original seal on the new locking system.
The conventional way of affixing a seal to the locking system is not entirely satisfactory from at least this view point. The identification mark is often on a flag which is separate from the aperture or apertures which receive the lock wire, so that it may be possible to disengage the lock wire from the apertures without comprising the integrity of the identification mark. As further drawbacks, adding a metal seal piece to the locking system increases the weight of the device, which is not desirable in aircraft applications. Mounting a seal on the locking wire and crimping the seal to engage the locking wire also add two more steps to the installation of the locking system, thereby increasing the effort required for system installation.